Practical aspects of urea SCR for automotive NOx control
Christine Lambert, Ford Motor Co.
Urea SCR is originally a steady-state technology used in stationary source NOx control, such as at refineries. The bed temperature may be controlled within an optimum conversion window, and the gas flow is fairly constant. Vehicle operation, on the other hand, is highly transient in temperature and flow based on customer demanded vehicle speed, load, road conditions, weather, etc., and may not be predicted. A catalyst must work in the vast majority of conditions found in normal vehicle use, and the size is limited by space and weight considerations. People drive vehicles, not catalysts, and emission control is not a feature highlighted in a marketing advertisement, so SCR operation must have negligible impact on drivability and require no user intervention beyond refilling the urea tank. Real time modeling & control schemes are required on vehicle to enable optimal urea dosing. The SCR catalyst functions and survives in a multi-component system that also controls hydrocarbons, carbon monoxide, and soot, and faces poisoning by sulfur, phosphorous, and a variety of other elements present in fuel, oil, and any upstream component including other catalysts. The SCR catalyst can experience temperatures below 200°C and as high as 700°C in typical vehicle operation, or even 900°C when coated on a soot filter. The selection of functional materials, durability tests representing high mileage, and even more important, tests to failure, become critical decisions in the design of automotive urea SCR. The subsequent flurry of research on Cu/CHA after commercial launch on vehicles is considered, plus a look at the future applicability of the technology and potential improvements.